Integrated circuits often include data converters such as analog-to-digital (A/D) converters. An analog-to-digital converter is a device that is configured to convert a continuous analog signal to a discrete-time digital signal. For example, an analog-to-digital converter can be used to convert an input analog voltage (or current) to a corresponding digital representation that is proportional to the magnitude of the input analog voltage.
There are many different types of analog-to-digital converters. As examples, the different implementations include flash A/D converters, single-slope A/D converters, dual-slope A/D converters, successive approximation A/D converters, pipeline A/D converters, oversampled A/D converters, etc. For applications in which compact die area and high speed operation are primary objectives, pipeline A/D converters are often used.
A pipeline A/D converter includes a series cascade of lower resolution pipeline stages. The different pipeline stages operate concurrently, where the first stage processes the most recent analog sample while each successive stage operates on the analog remainder voltage (often referred to as a residual voltage) output from the previous pipeline stage.
A general pipeline stage may include a sample-and-hold (S/H) circuit, a low resolution A/D converter such as a low resolution flash A/D converter, a low resolution digital-to-analog (D/A) converter, a subtraction circuit, and a finite gain operational amplifier. Each stage uses the S/H circuit to sample and hold the residual voltage received from its previous stage. The low resolution flash A/D converter is configured to compare the held input (i.e., the held residual voltage received from the previous stage) with fixed threshold voltage reference levels.
With the low resolution A/D converter, the held input is converted into a low resolution digital signal. The low resolution digital signal is then converted back to analog using the low resolution D/A converter. This converted signal presented at the output of the low resolution D/A converter is subtracted from the held input to obtain a difference voltage using the subtraction circuit. This difference voltage is amplified using the finite gain operational amplifier to produce a corresponding residue voltage that is passed to the next stage. In some pipeline stage designs, the sample-and-hold, subtraction, and gain functionalities are integrated into a single switched capacitor circuit to reduce area overhead.
Because each of the pipeline stages operates simultaneously, the number of pipeline stages (which determines the resolution of the pipe A/D converter) is not limited by target throughput criteria. The resolution of a pipeline A/D converter may, however, be limited to no more than eight bits due to inter-stage gain error (i.e., error in the gain of the operational amplifier in each stage), variations in the amplifier gain, capacitor mismatch among the switched capacitor circuits, variations in the threshold voltage reference levels, and charge injection from S/H switches, just to name a few.
Conventional pipeline A/D converter calibration techniques rely on complex algorithms to help mitigate such types of systematic errors but may require substantial area overhead and may be unacceptably time consuming.